1. Field of the Invention
The present invention relates to drive units having an overcurrent protection function for driving switching elements in an inverter, for example to be applied to motor generators, etc.
2. Description of the Related Art
There is a known technique, for example, patent document 1, Japanese patent No. 3367699, which has disclosed a conventional drive unit capable of driving a semiconductor switching element, for example composed of Insulated Gate Bipolar Transistors (IGBTs). The drive unit has an overcurrent protection function capable of decreasing a gate voltage of the semiconductor switching element when a collector current, which flows between an input terminal (the collector terminal) and an output terminal (the emitter terminal) of the semiconductor switching element, exceeds a threshold value. This operation forcedly cuts off the collector current.
However, when such an overcurrent protection function is applied to a drive unit in an electric power conversion circuit such as a three phase inverter, there is a possibility of causing the following drawback. In this case, the three phase inverter is equipped with series connection units. Each of the series connection units is comprised of a switching element in a high voltage side and a switching element in a low voltage side.
For example, under a condition in which one of the switching element in a high voltage side and the switching element in a low voltage side in each of the series connection units enters a completely turned-on state (or a full turned-on state), and an upper-lower arm short circuit occurs, in which the other switching element, which is not turned on, suffers a short circuit defect, when an overcurrent protection function forcedly cuts off the collector current flowing between the input terminal and the output terminal of the semiconductor switching element, this has a possibility of generating a large surge voltage. Such a large surge voltage often deteriorates the operational reliability of the switching element in a high voltage side and the switching element in a low voltage side.